Fennec 1.1a2pre
If you have zoomed into a page at least by one level with the volume keys on your N900 device and switch from landscape mode to the portrait mode and back again, the tab sidebar is always shown.
Steps:
1. Open https://litmus.mozilla.org
2. Press the Vol+ button to zoom in
3. Switch to portrait mode
4. Switch back to landscape mode
After step 4 the tab sidebar will always be displayed.